For storage of data, materials and devices with reversible optical storage capability are needed. Optical storage can be applied in optical computers, storage systems for audio and video informations.
There are many types of optical storage devices, which can be written in only once (write-once-read-many systems=WORM systems), using irreversible processes for storage. However they were not applicable for storage systems for computers and, find only limited application in audio and video informations because of their non-reversible properties.
For many purposes more advantageous are devices, which can be written in multifold (erasable-direct-read-after-write systems=EDRAW systems) by use of reversible storage processes. For this purpose a survey about materials for optical data storage has been presented by M. Emmelius, G. Pawlowsky and H. W. Vollmann, Angew. Chem. Intern. Ed. 28, 1445 (1989). However because all available media for optical data storage have some shortcomings like storage density, storage rate, reversibility, long time stability, the search for new optical storage materials and devices is quite actual.
Devices of this kind typically consist of a layer of liquid crystal material, contained between two glass slides. By irradiation with light of different wavelengths, devices containing azo compounds change from the nematic to the isotropic state and reverse (for example, D. Demus, G. Pelzl, F. Kuschel DD WP 134 279).
Especially liquid crystalline polymers containing azo groups have been proposed for such devices (H. Finkelmann, W. Meier and H. Scheuermann, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific Singapore 1992, vol. 3, p. 345-370).
The proposed azo compounds have the disadvantage, that the intensity and time of irradiation are quite high, because a large amount of the azo compound has to be transformed to the corresponding isomer. Therefore the switching times are extremely large.
Also thermo-optical liquid crystal devices have been proposed, based on thermally induced texture change of cholesteric phases (S. Kobayashi and A. Mochizuki, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1992, vol. 3, p. 291-293) or smectic A phases (D. Coates, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1990, vol. 1, p. 275-303).
Thermo-electrooptic displays use the combined effect of heat and electric fields, in glass forming low-molecular glass forming liquid crystals (D. Demus and G. Pelzl, DD WP 242 624 A1) resp. polymer glass forming liquid crystals (H. Finkelmann, W. Meier and H. Scheuermann, in: Liquid Crystals. Applications and Uses, ed. by B. Bahadur, World Scientific, Singapore 1992, vol. 3, p. 345-370). However thermo-optical devices generally need a quite large energy for locally heating up the liquid crystal material.
M. Zhang and G. B. Schuster, J. Phys. Chem. 96, 3063-3067 (1992) reported about the photoracemization of chiral binaphthyl derivatives, converting the cholesteric mixture to nematic mixture. Because the reaction is irreversible, the change from nematic to cholesteric is impossible and the material cannot be used for repeated data storage.
Holographic technology has been investigated for reversible data storage systems. Several kind of materials have been investigated toward this application. There are three types of materials, inorganic metal compounds, photorefractive polymers and polymer liquid crystals, have been reported. However they all have disadvantages.
For materials of optical data storage, photorefractive crystals like lithium niobate (LiNbO.sub.3), barium titanate (BaTiO.sub.3) and bismuth silicon oxide (Bi.sub.12 SiO.sub.20), have been proposed and studied for 25 years. However because they are difficult and expensive to grow, and their properties cannot easily be modified, they have not become commercially feasible.
Variety of photorefractive polymer and polymer liquid crystals have also been proposed (K. Yoshinaga et al., Japanese Patent Kokai 2-280116, T. Todorov et al., Applied Optics, 23 (23), 4309 (1984), M. Eich et al., Makromol. Chem., Rapid Commun., 8, 59 and 467 (1987)). It was recognized that the photorefractive effect enable them to store 100 or more complete image or holograms, each containing 1 million bits of imformations. They can be easily produced so that they have a big economic advantage. They, however, have significant disadvantages. The speed with which they respond to light is quite slow compared with semiconductors. And most of the existing polymers require that an electric field be applied to orient the non-liner optical chromophores so that the material becomes electro optically active.
Novel data storage systems, devices and materials which are easy to produce, and have advanced properties, such as low viscosity facilitating quick response, large capacity of data storage, reversible and stable with reading and writing informations, have been desired. The aim of this invention is to overcome the above mentioned disadvantages of the previous technologies and providing novel data storage systems, devices and materials having above mentioned desired properties.